This invention generally relates to a process and an apparatus for effecting the development of color images. More specifically the present invention is directed to an improved process, and an improved apparatus for use in a single pass two-color development system, wherein contamination of the color developer is substantially eliminated. In one embodiment of the present invention there is provided in the development process and apparatus a magnet system which enables the removal of undesirable black toner particles from the color developer mixture. There is thus provided in an embodiment of the present invention an improved process, and an improved apparatus for obtaining color images by providing development zones encompassed by a moving deflected flexible imaging member and a moving transporting member, and wherein at one of the developer stations there is situated a magnet system for the purpose of removing black toner particles from the color developer composition reservoir thereby preventing contamination thereof. In this manner the process and apparatus of the present invention provides for the continual uncontaminated development of high quality images, particularly images of different colors, such as black and red, including the efficient and effective development of solid areas. Therefore, the problem of causing undesirable contamination of the colored toner composition black toner composition, for example, is uniquely solved in accordance with the process and apparatus of the present invention by positioning a magnet within the development apparatus.
The development of images by electrostatographic means is well known. Generally in these processes toner particles are applied to an electrostatic latent image by various methods including cascade development, reference U.S. Pat. 3,618,552; magnetic brush development, U.S. Pat. Nos. 2,874,063; 3,251,706 and 3,357,402; powder cloud development, U.S. Pat. No. 2,217,776; and touchdown development, U.S. Pat. No. 3,166,432. Cascade development and powder cloud development are especially well suited for the development of line images common to business documents, however, images with solid areas are not faithfully reproduced by these methods. Magnetic brush development systems, however, provided an improved method for reproducing both line images and solid areas.
In magnetic brush development systems, it is usually desirable to attempt to regulate the thickness of the developer composition, which is transported on a roller, by moving the roller past a metering blade. The adjustment of the metering blade is important, since in the development zone the flow of developer material is determined by a narrow restrictive opening situated between the transport roller and the imaging surface. Accordingly, in order to provide sufficient toner particles to the imaging surface, it is generally necessary to compress the developer bristles, thereby allowing toner particles adhering to the carrier particles near the ends of the bristle to be available for development. Any variation, or non-uniformity in the amount of developer metered onto the transport roller, or into the spacing between the transport roller and imaging member can result in undesired developer flow, and non-uniform image development. Non-uniform development is usually minimized by carefully controlling developer runout on the transport roller, and on the imaging member, and by providing a means for side-to-side adjustment in the relative positions of the metering blade, development roller and imaging member.
While several improved toner and carrier materials as well as processes have been developed for the purpose of obtaining images, many difficulties existed in designing a simple, inexpensive and reliable two-component development system which will provide high solid area development rates, low background deposition, and long term stability. This was accomplished with the process and apparatus as described in U.S. Pat. No. 4,394,429 entitled Development Process and Apparatus, the disclosure of which is totally incorporated herein by reference. There is illustrated in this patent a self-agitated two-component development process and apparatus wherein toner is continuously available immediately adjacent to a flexible deflected imaging member, and toner particles transfer from one layer of carrier particles to another layer of carrier particles in a development zone. More specifically, in one embodiment of the invention described in the 4,394,429 patent there is described an improved process for causing the development of electrostatic latent images on an imaging member comprising providing a development zone, ranging in length of from about 0.5 centimeters to about 5 centimeters, encompassed by a tensioned deflected flexible imaging member and a transporting member, wherein the flexible imaging member is comprised of a supporting substrate, a photogenerating layer, and a diamine hole transport layer. The deflected flexible imaging member and transporting member can be caused to move in opposite directions at certain specific speeds; and furthermore, there is usually maintained a distance of from about 0.05 millimeters to about 1.5 millimeters, between the flexible imaging member and the transporting member.
Furthermore, it is known that color images can be obtained in xerographic imaging systems. Specifically, two-color reproduction systems serving distinct needs are generally known; representational wherein the colors of the reproduction are equivalent to those of the original document; and functional wherein the color to be reproduced merely serves to mark, distinguish or highlight portions of a document such as a text, graphs, or line drawings. In representational color processes, images are xerographically produced, for example by three successive color filter exposures, followed by an in-register transfer of toner images produced by three toners of the appropriate primary attractive colors. These processes are complex in that they require the superimposition of images on three separate exposures, either in three successive cycles, or on a photoreceptor of sufficient circumference or length to accommodate the images prior to transfer. Also, it is known to use in such processes a series of three separate in-register photoreceptor drums each contributing one image to the final transfer sheet. However, these processes can be costly, and can result in images of poor resolution in view of their complexity, and the necessity of using three separate photoreceptor drums. In the simpler highlight color imaging systems, to which the present invention relates, generally only two colors need to be reproduced. Specifically, in highlight color processes there are produced two color documents wherein, for example black may be used to represent the main text, and red, blue, or other color selected portions of the text, which are to be directed to the users attention by means of the highlight color. Accordingly, there can be generated images in two colors, such as red and black, by desirably employing only one imaging operation.
Moreover, there is disclosed in U.S. Pat. No. 4,188,213, a method for producing color copies involving a number of complex steps including, for example, the recording of successive single color electrostatic latent images on an image bearing member, followed by developing each color electrostatic latent image with particles containing a predetermined dominant colorant therein corresponding to each recorded single color electrostatic latent image, transferring layers of the developed particles to a sheet of support material, and regulating electrically the transfer step in order that the successive thinner layers of particles are transferred from the image bearing member to a sheet of support material. Disclosed in this patent is a development method involving the deposition of particles containing a dominant cyan colorant with a minor magenta colorant impurity, on an electrostatic latent image formed from a red filtered image, depositing particles with a dominant magenta colorant with a minor yellow impurity on the electrostatic latent image formed from a green filtered light image, and depositing particles with a dominant yellow colorant on the electrostatic latent image formed from a blue filtered light image. Each successive layer of toner particles, which are transferred to a sheet of support material, is of a color corresponding to the color of impurity in the previously transferred layer of toner particles. Thus, successive layers of toner particles are transferred in superimposed registration with one another, with each successive transferred layer of toner particles correcting for the impurities in the colorant of the previously transferred layer of toner particles, thus producing a combination of toner particles substantially approximately the desired color.
The invention disclosed in U.S. Pat. No. 4,189,224 solves some of the problems of the process of the 4,188,213 patent, in that the method described therein requires only a single exposure to derive a two color image. Therefore, registration and multiple cycling steps are eliminated. More specifically, there is described in U.S. Pat. No. 4,189,224 a two color electrostatic copying apparatus which can be operable for one color positive or negative copying. In accordance with the teachings of this patent, a photoconductive material with a conductive substrate, an inner photoconductive layer sensitive to visible light, and an outer photoconductive layer insensitive to red light, is subjected to an electrostatic charge applied to the outer layer, while at the same time irradiating the device with light so as to render one of the layers conductive. Subsequently, an electrostatic charge of opposite polarity is applied to the outer layer of the photoresponsive member, this step being accomplished in the dark. A light image of an original document is then projected onto the outer layer of the photoresponsive device, wherein white areas of the image cause photoconduction of both layers and red areas thereof, enabling photoconduction of only the inner layer. As a result, white areas are of zero surface potential, while red and black areas have non-zero surface potentials of opposite polarities. The images can then be developed with red and black toner particles of opposite charge. Thus, for example, red particles which are charged positively will be caused to adhere to negatively charged image areas, while black toner particles which are charged negatively will adhere to the black image areas which are charged positively.
Also, disclosed in U.S. Pat. No. 4,078,929 is a single step electrostatographic copying process in which two different potential levels on a photoresponsive device may be developed in immediate sequence subsequent to a single exposure, by means of two differentially colored xerographic toners. The two potential levels may be of the same polarity or preferably of opposite polarities. In one embodiment of the invention disclosed in the 4,078,929 patent, positively charged toner particles of a first color, and negatively charged toner particles of a second color are about evenly concentrated in the relatively negative, and relatively positive areas of the imaging surface, thus allowing the positively charged toner particles to be attracted to the imaging surface with a negative charge pattern, while the negatively charged toner particles of a second color are attracted to the imaging surface with a positive charge pattern.